The invention relates to a method for heating a roll, and in particular to a method of heating a roll in which the roll is heated at different points along the axial length of the roll to achieve a heating profile along the roll. The invention also relates to a device for heating a roll, and in particular to a device for heating a roll which is equipped with a profiling induction heater. Furthermore, the invention relates to a method for rebuilding a heating system of a roll, and in particular to a method for rebuilding a heating system in which the existing heating system includes a profiling induction heater.
In paper or paperboard machines or finishing machines for paper or paperboard, rotating rolls are used for treating the paper web. Such rolls are used especially in calenders, wherein linear load and/or heat is exerted on the web passing by the roll to treat the web in the desired manner. The calender may be placed either in the production line of paper, wherein it treats the web coming from the drying section of the paper machine, or it may be located in a separate paper finishing machine, wherein the processed paper web is unwound from reels. Other rolls that treat the web by means of heat and/or pressure, include rolls of the press section and drying cylinders of the drying section.
The calender roll is arranged rotatable in the frame of the calender in such a manner that it forms a so-called calender nip with the moving surface of a counter element, wherein the paper web to be processed is guided through this nip. The counter element on the other side of the nip may be another rotating calender roll but also a continuous belt passed via a roll or a stationary supporting surface. In its simplest form, the calender may be formed of one nip, but it may also consist of two or more nips, which each can be formed between a calender roll and an opposite moving element. To produce successive nips in the travel direction of the web, the pairs of a calender roll and a counter element may be separate units in the frame of the calender, or a so-called roll stack may be formed of the calender rolls, wherein the web travels along a winding path via the nips formed between the rolls.
The calendering nip may be formed between two hard surfaces, for example between two smooth-faced metal rolls, or between a hard surface and a soft surface, wherein the latter is typically attained with a soft cover in a metal-faced roll or by means of an elastic belt passed over the roll or a stationary shoe element.
It is common in all the aforementioned solutions to heat a metal-covered roll, and there are many alternatives for heating the roll, such as a heating medium fed inside the roll, radiation heating by means of heating elements outside or inside the roll, or induction heating by means of a magnetic field with induction coils arranged inside or outside the roll.
Examples of induction heating are disclosed for example in Finnish patent 71375 and in the corresponding U.S. Pat. No. 4,614,565, Finnish publication 74825 and in the corresponding U.S. Pat. No. 4,384,514 as well as in the European patent 196 264. These publications disclose induction heating by means of electromagnetic coils i.e. induction coils arranged outside the shell of the roll. It is also possible to conduct the heating by controlling each roll separately, wherein temperature profiling can be attained, by means of which it is also possible to affect the nip profile through thermal expansion of metal. U.S. Pat. No. 4,384,514 presents coils arranged in two rows which are placed in a staggered relationship so that their areas of influence cover the the heated width of the roll shell, to perform the heating in a profiled manner by adjusting each coil separately.
An induction heater that is arranged inside a rotating roll and exerts a magnetic field on the shell of the roll is, in turn, disclosed in U.S. Pat. Nos. 4,425,489, 5,074,019 and 5,895,598. The electromagnetic coils located in the induction heater may be independently controllable to perform the induction heating in a profiled manner. U.S. Pat. No. 5,074,019 presents a solution, in which the shell of the roll is heated by induction heaters both inside and outside, and the aim is thus to provide as even heating as possible and to avoid an uneven surface temperature of the roll (column 5, lines 67 to 68, of the publication).
Furthermore, U.S. Pat. No. 6,289,797 and corresponding German patent application 19911963 disclose the possibility of placing zonewise controlled induction coils inside a polymer-coated calender roll.
European patent 277905 discloses a non-profiling external induction heater in connection with a calender roll equipped with an internal heating medium.
Thus, electromagnetic coils, i.e. induction coils are commonly used for heating of the outer surface of rotating rolls in a paper machine or a finishing machine for paper up to a fixed temperature by producing eddy currents in the shell of the roll by means of induction, said eddy currents heating the shell of the roll in such a manner that the outer surface of the shell that is in contact with the web, reaches a predetermined temperature.
Thus, it is well-known to use induction heaters for heating calender rolls in such a manner that, as a result of locally adjusted thermal expansion of the shell of the roll, the desired nip profile and thereby the adjustment of the thickness profile of paper passed through the nip is attained. Profiling induction heaters, which are disclosed for example in the aforementioned publications, are also well-known. Furthermore, it is known to use such induction heaters for profiling heating of calender rolls which are equipped with an internal heating medium.
In calenders as well as in other possible objects in a paper or paperboard machine or in a finishing machine for paper or paperboard, in which it is possible to use induction heating of the roll, there is a lack of space due to the compact structure and the lay-out and the variety of auxiliary devices required in the process. Therefore, it is of primary importance that the profiling induction heater can, first of all, be constructed to be very compact.
It is an aim of the invention to present a method whereby the above-mentioned disadvantages can be eliminated in such a way that the roll can be effectively heated with solutions which take little space.
In the method according to the invention the basic heating of the roll is effected by unprofiling heating outside the roll. In addition to this, the same roll is subjected to profiling induction heating. In a calender application, this profiling heating can be coupled to an automatic caliper adjustment (adjustment of the nip profile). Previously, profiling induction heating has also been used for basic heating, wherein it has been necessary to make the profiling induction heater large.
Another aim of the invention is to introduce a device by means of which it is possible to implement precise heating of the rolls with induction heaters.
The device according to the present invention comprises both a non-profiling heater outside the roll, which heater achieves even basic heating of the roll, and a profiling induction heater. Both heaters are preferably placed outside the roll, one after the other in the direction of rotation of the roll. Each heater can be controlled and adjusted separately. The primary variable to be controlled/adjusted for the non-profiling heater is a given heating effect, and the variable to be controlled/adjusted for the profiling heater is a given temperature profile. The non-profiling heater can be integrated in the same compact support structure, e.g. a beam structure, as the profiling heater.
The non-profiling even heater is preferably also an induction heater. In this way, it is possible to completely eliminate heating by means of a heat transfer medium, such as hot oil, which requires tube arrangements in the roll structure. Similarly, it is easier to implement the adjustment and control when the heatings of the roll are effected by the same principle.
In the method for rebuilding a system for heating a roll according to the invention a heating system which already comprises a profiling induction heater is provided with a supplementary non-profiling heater. If necessary, it is also possible to replace the roll itself; for example, a roll equipped with the circulation of a heating medium can be replaced with a roll of another type which does not contain internal heating medium channels.